Small centrifugal heat pump

ABSTRACT

A variable capacity mechanical refrigeration system for heat pump or cooling operation, with a variable speed centrifugal compressor motor drive that uses an electronic frequency conversion apparatus which is sensitive to and controlled by, discharge or suction pressure and which includes means of preventing overloading during start up of the compressors.

United States Patent Ruff et al.

[ SMALL CENTRIFUGAL HEAT PUMP [451 Sept. 19, 1972 3,355,906 12/1967Newton ..62/228 [72] Inventors: John D. Ruff, 206 Birch St.; Phillip R.Wheeler, 209 Pine St., both of Pr1mwwExaminerMeyerPerlm Alexandria, Va.22305 22 Filed: May 15,1970 [57] ABSTRACT [21] APPLNQ; 37,779 A variablecapacity mechanical refrigeration system for heat pump or coolingoperation, with a variable speed centrifugal compressor motor drive thatuses an jll ..62/2I;( )2,56b2/l5/l)3 electronic frequency conversionapparatus which is [58] Fieid 230 215 sensitive to and controlled by,discharge or suction 2 6 pressure and which includes means of preventingoverloading during start up of the compressors. [56] Rderences c'ted 2Claims, 3 Drawing Figures UNITED STATES PATENTS 3,324,672 6/1967 Sones..62/228 l IZEQV. 4-1:.

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FIG. 1

INVENTOR SMALL CENTRIFUGAL HEAT PUMP This invention is similar in manyrespects to our earlier inventions, US. Pat. Nos. 3,449,922, 3,447,335,and 3,499,297. Similarly to those inventions the object is to produce anelectrically powered mechanical refrigeration system using hermetic,kinetic displacement compressors centrifugal or axial flow) and whichhas very efficient capacity variation capabilities. As with our otherinventions this capacity variation is achieved by variable speed drivingof the compressor system. This is done by variable frequency conversionof the electric power supplied from an external source such aselectrical power mains or any other source of electrical power. Thecompressor motor used by this invention is of the squirrel cageinduction type and its speed is dependent on the alternating frequencyof the electric current supplied to it. This type also has no brushesand can be used in hermetic type systems. Hermetic systems of course donot use compressor shaft seals and with small high speed compressorsthis is very desirable.

This invention is mainly concerned with the variable speed drive to thecompressor system and the control means associated therewith. Suchvariable speed drives can be applied to all sizes of kineticdisplacement compressors, and in all kinds of application (heat pump,air conditioning, refrigeration).

However this invention relates primarily to the methods of varyingcompressor capacity which depend for their control on the systemdischarge pressure, with the system being used as a heat pump, or on thesystem suction pressure when the system is being used for coolingpurposes.

Methods are also included which prevent compressor overloading at startup. This is done by building up the system discharge pressure prior toapplying normal compressor speed control or by restricting vapor flowthrough the system by a throttling device for a predetermined time oruntil the discharge pressure has built up.

This invention comprises:

A variable capacity, variable frequency, hermetic, mechanicalrefrigeration system using kinetic displacement (centrifugal or axialflow) compressor machinery and driven by a squirrel cage motor (ormotors), and a variable frequency inverter to supply current to themotor (or motors).

A capacity controller, to control the inverter frequency, which issensitive and responsive to the system discharge pressure with thesystem being used as a heat pump.

A time delaying device which (on system start up) over-rides thisdischarge pressure sensitive capacity controller and maintains a minimumcompressor speed for a pre-set time and which can also be used tode-activate the condenser fan for this preset time so that the systemdischarge pressure is established more readily.

An alternate means of restricting refrigerant vapor flow, comprising athrottling device which restricts the main flow of refrigerant vaporthrough the system (during start up) and which can be disengaged by atimer.

A current sensing device which is an alternate means of disengaging thethrottling device (after start up), when the discharge pressure hasbuilt up and the motor current has dropped to a present level.

A capacity controller, to control the inverter frequency, which issensitive and responsive to the system suction pressure with the systembeing used for cooling purposes.

A timing device which (on system start up) overrides this suctionpressure sensitive capacity controller and maintains a minimumcompressor speed for a preset time and which can also be used tode-activate the evaporator fan for this preset time so that the systemsuction pressure is established more readily.

An alternate means of restricting refrigerant vapor flow, comprising athrottling device which restricts the main flow of refrigerant vaporthrough the system (during start up) and which can be disengaged by atimer, (when the system is being used in cooling function).

A current sensing device which is an alternate means of disengaging thethrottling device (after start up), when the suction pressure hasstabilized and the motor current has dropped to a preset level.

An alternate type of system which uses a dry expansion evaporatorandwhich does not need any device for preventing overloading at start updue to the operating characteristics peculiar to this type of system.

In the drawings:

FIG. 1 shows system layout in heat pump operation.

FIG. 2 shows the system layout when a dry expansion evaporator is used.

FIG. 3 shows system layout in cooling operation.

COMPRESSOR FIG. 1 shows compressor 2 which is a two stage centrifugalcompressor with hermetic enclosure 3.

FREQUENCY CONVERTER FIG. 1 illustrates, in simple form, a high frequencyconverter of typical specifications for supplying power to thecompressor motor. The 220 v. A.C., 60 Hertz (cycles per second), singlephase supply is first rectified by the bridge rectifier 4 using solidstate diodes 5,6,7,8. This pulsating D.C. output is smoothed by filtercapacitor 9 and fed to the silicon controlled rectifiersl0,ll,12,l3,l4,15 which switch into the three phase motor windings16,17,18 of the motor. Some filtering may be necessary at the windingsto achieve optimum wave shape and can be provided as needed. The firingof the silicon controlled rectifiers 10,1 1,12,13,14,]5 is controlled bythe variable frequency phase sequencer (or trigger circuit) 19. Thiscircuit triggers and turns off the silicon controlled rectifiers in thenecessary sequence for a three phase operation. The frequency of theresulting three phase supply is variable by changing the frequency ofthe sequencer oscillations.

By this method the speed of rotation of the compressor motor iscontrolled since this motor is of the squirrel cage induction type andits speed is dependent on its supply frequency/Since the capacity of acentrifugal compressor varies basically as the cube of the rotationspeed, then the operating capacity of the compressor unit will beapproximately reduced to 40 percent of full capacity by a 25 percentdrop in the rotation speed. This would be a typical range of capacityvariation, with a span of about 40 F. in evaporator temperature (0 F. 40F.). Capacity in this case refers to the work done by the compressor asexpressed in horsepower. Actually this capacity is dependent verylargely, on the pressure against which the compressor is working (or thecompression ratio between the condenser and evaporator pressures).

There are several methods 'of achieving the frequency conversion tocontrol motor speed but an electronic converter as described above ispreferred since the necessary frequency changes can be accomplishedsimply by changing the frequency in the trigger circuit 19.

HEAT PUMP OPERATION FIG. 1 shows compressor 2 drawing vapor fromevaporator coil 20 which is in contact with the outside air and thusprovides a source of heat. Condenser coil 21 is contacting the airinside the heated space and is thus heating it. Controller 22 is apressure sensitive device which is the means of controlling thefrequency of sequencer 19 and is sensitive to the condensing pressure incondenser 21 to which it is connected by line 23. That is, when theoutside air temperature is lowered the suction pressure in evaporator 20is also lowered, and since the pumping head of compressor 2 isrelatively constant at any given rotation speed, then the condensingpressure will also be lowered. Controller 22 senses this drop inpressure and causes the frequency of sequencer 19 to be increased. Thisspeeds up the compressor and increases its pumping head and thecondensing pressure is thus kept at a reasonable level. It is necessarythat the condensing pressure be kept at a reasonable level so that thecondensing temperature also can be dept high and thus the heatingfunction of the heat pump maintained. Similarly an undesirable raisingof condensing pressure (due to warmer outside conditions) is compensatedby a slowing down of the compressor. Controller 22 is shown withswitches 24,25,26 which are actuated, a step at a time, by movement ofbellows 27 as it responds to varying condensing pressures. Switches 24,25, 26 short out portions of resistor 28, the resistance of which is thebasis of control of sequencer 19 as was explained in more detail in ourearlier invention (US. Pat. No. 3,449,297). This arrangement ofcontroller 22 gives four steps of capacity which are engagedautomatically in response to pressure variations.

Alternative arrangements are the use of water flowing over evaporatorcoil 20 (as a source of heat) and the use of water flowing overcondenser 21 (with a hydronic circulation system). Controller 22operates in the same manner with these arrangements.

OVERLOAD PREVENTION When the centrifugal (or axial flow) compressor isstarting up after being stopped for any length of time there is atendency to overload because the pressures in evaporator 20 andcondenser 21 are nearly equal and there is a high rate of flow throughthe compressor. However, (with an air to air system) the evaporator sooncools down and the condenser heats up and normal operation is allowed.Our invention uses a timer 29 which over-rides the switches 24, 25, 26for a preset time when the system is started up thus causing thecompressor to run at the minimum speed and thus minimize overloading. Atthe end of the preset time, timer 29 disengages and normal operation isresumed.

An alternative method of reducing the rate of flow through thecompressor is by the use of throttle valve 30 which can be placed ineither the suction line or the discharge line of the compressor. It iselectrically activated and controlled by timer 31 during a preset timeduring starting up of the system. A bypass 32 is used so that a reducedflow can be maintained while the throttle is engaged. Means are shownfor switching either of time rs 29 or 31 into the control circuits.

During the engagement of the overload prevention methods described amore positive action is obtained if the evaporator fan 33 and thecondenser fan 34 are turned off by the timers. This allows thetemperature change (and thus pressure change) in coils 20 and 21 to bemore rapid. An alternative to the use of timers is the use of slowacting mechanisms that actuate the throttle valve 30 or the capacitycontroller 22.

An alternate method of controlling throttle valve 30 is by the use ofmotor current actuated terminator 35. On start up, throttle valve 30 isengaged and its disengagement is controlled by terminator 35. Pick-upcoil 36 detects the level of motor current flowing in one of the motorleads, and when this has dropped to a pre-set level a relay interminator 35 is actuated and throttle valve 30 is disengaged. Thismethod is most suitable for hydronic systems which take varying andsometimes much longer times to settle down on start up. A timer wouldnot be applicable in this case. Means are shown for switching thisalternate terminator into the control circuit.

DRY EXPANSION SYSTEM FIG. 2 shows a heat pump system using a dryexpansion evaporator 37. Controller 22 is used to control capacity inthe manner already described. However no problems are encountered withstart up overloading since there is no quantity of liquid refrigerant inthe evaporator at start up as with a flooded evaporator. However specialprecautions should be taken such as mounting liquid receiver 38 at alevel lower than evaporator 37 and pitching the tubes of the evaporatorso that any liquid in the evaporator will drain back through expansionvalve 39 into the receiver. Evaporator 37 can be contacted by a flow ofoutside air, or water can alternately be used as a source of heat.

An alternate method of keeping the evaporator 37 free of refrigerantliquid is by a pump down system. When the system operation is to bestopped, solenoid 40 is de-energized and the flow of refrigerant liquidto evaporator 37 is stopped. Compressor 2 continues to run and all ofthe refrigerant in evaporator 37 is drawn out. The suction pressure thendrops and low pressure switch 41 is opened and this switch deactivatesthe compressor motor. Check valve 42 closes and holds the systemrefrigerant from flowing back through the compressor and into theevaporator 37. Should some vapor leak through check valve 42 or solenoid40 the pressure at switch 41 causes it to close and the compressor pumpsdown again. Condenser 21 can be air contacted, or, alternately it can bewater contacted (with hydronic circulation).

COOLING OPERATION FIG. 3 shows the invention in use for coolingoperation. Operation is similar to heat pump operation.

Compressor 2 pumps from evaporator into condenser 21. However forcooling operation the controller 43 is sensitive to the suction pressurein evaporator 20 to which it is connected by line 44. And also theaction of the controller 43 is the reverse of controller 22. That is, ittends to speed up the compressor on rise of pressure rather than on dropof pressure as with controller 22. Switch points 45,46,47 make on fallof pressure. The action of timer 29 in delaying full capacityapplication by over-riding controller 43, and the action of throttle 30controlled by timer 31 and terminator 35 are the same as with heat pumpoperation. Also a hydronic circulation can alternately be used throughevaporator 20, and condenser 21 can alternately be water cooled.

When the system is used for cooling with a dry expansion evaporator(FIG. 2) the operation is similar to heat pump operation but controller43 is then used instead of Controller 22. Controller 43 is connected tosuction pressure in the same manner as in FIG. 3 and in similar functionit is reverse acting. Means are shown for switching this alternatecontroller into the control circuit. Evaporator 20 can be contacted by aflow of inside air or, alternately a hydronic system can be used.Condenser 21 can be air cooled or it can alternately be water cooled.

We claim:

1. In combination a variable capacity heat pump used for heating an airfilled space and comprising kinetic displacement vapor compression meansdirect driven by variable frequency, alternating current, electric motormachinery of the squirrel-cage induction type and the said compressionmeans and motor machinery being combined in a common hermetic enclosure,to pump refrigerant vapor from a dry expansion evaporator to acondenser, a variable frequency inverter means using sequentialswitching equipment to switch electrical current from an external sourceinto the windings of the said motor machinery in such a manner toprovide a variable frequency flux through the said windings, and thusachieve a variable speed rotation of the said motor machinery, since therotation speed of this type of motor is dependent on the frequency ofthe current supplied to it, a pressure sensing means which is sensitiveonly to the refrigerant vapor pressure in the said condenser, and thefrequency of the said variable frequency inverter is controlled by thissaid pressure sensing means so that when operating conditions cause thepressure in the said condenser to be undesirably low the compressor isspeeded up until a suitable condenser pressure is reached and similarlywhen the condenser pressure is too high the compressor is slowed down.

2. In combination a variable capacity cooling system used for cooling anair filled space and comprising kinetic displacement vapor compressionmeans direct driven by variable frequency, alternating current, electricmotor machinery of the squirrel-cage induction type and the saidcompression means and motor machinery being combined in a commonhermetic enclosure, to pump refrigerant vapor from a dry expansionevaporator to a condenser, a variable frequency inverter means usingsequential switching equipment to switch electrical current from anexternal source into the windings of the said motor machiner in such amanner to provide a variable frequency ux through the said windings, andthus achieve a variable speed rotation of the said motor machinery,since the rotation speed of this type of motor is dependent on thefrequency of the current supplied to it, a pressure sensing means whichis sensitive only to the refrigerant vapor pressure in the saidevaporator, and the frequency of the said variable frequency inverter iscontrolled by this said pressure sensing means so that when operatingconditions cause the pressure in the said evaporator to be undesirablyhigh the compressor is speeded up till a suitable evaporator pressure isreached and similarly when the evaporator pressure is too low thecompressor is slowed down.

1. In combination a variable capacity heat pump used for heating an airfilled space and comprising kinetic displacement vapor compression meansdirect driven by variable frequency, alternating current, electric motormachinery of the squirrelcage induction type and the said compressionmeans and motor machinery being combined in a common hermetic enclosure,to pump refrigerant vapor from a dry expansion evaporator to acondenser, a variable frequency inverter means using sequentialswitching equipment to switch electrical current from an external sourceinto the windings of the said motor machinery in such a manner toprovide a variable frequency flux through the said windings, and thusachieve a variable speed rotation of the said motor machinery, since therotation speed of this type of motor is dependent on the frequency ofthe current supplied to it, a pressure sensing means which is sensitiveonly to the refrigerant vapor pressUre in the said condenser, and thefrequency of the said variable frequency inverter is controlled by thissaid pressure sensing means so that when operating conditions cause thepressure in the said condenser to be undesirably low the compressor isspeeded up until a suitable condenser pressure is reached and similarlywhen the condenser pressure is too high the compressor is slowed down.2. In combination a variable capacity cooling system used for cooling anair filled space and comprising kinetic displacement vapor compressionmeans direct driven by variable frequency, alternating current, electricmotor machinery of the squirrel-cage induction type and the saidcompression means and motor machinery being combined in a commonhermetic enclosure, to pump refrigerant vapor from a dry expansionevaporator to a condenser, a variable frequency inverter means usingsequential switching equipment to switch electrical current from anexternal source into the windings of the said motor machinery in such amanner to provide a variable frequency flux through the said windings,and thus achieve a variable speed rotation of the said motor machinery,since the rotation speed of this type of motor is dependent on thefrequency of the current supplied to it, a pressure sensing means whichis sensitive only to the refrigerant vapor pressure in the saidevaporator, and the frequency of the said variable frequency inverter iscontrolled by this said pressure sensing means so that when operatingconditions cause the pressure in the said evaporator to be undesirablyhigh the compressor is speeded up till a suitable evaporator pressure isreached and similarly when the evaporator pressure is too low thecompressor is slowed down.